One function of rotary drill bits is use in blast hole drilling. In general, blast holes have a depth in the range of about 50 to about 150 feet and are filled with a blasting material for breaking up the earth during mining operations. The body of the drill bit typically used for drilling blast holes is attached to a drill pipe by a threaded member on the body of the bit. The drill pipe is supported and rotated by a drilling rig. The body of the drill bit typically has three legs, each of the legs having a projecting, conical cutter-receiving journal. Three conical cutters, each having an axially extending recess open at one end, are rotatably mounted on respective journals with the use of friction reducing bearings interior to the conical cutters. Each conical cutter has rock cutting teeth or inserts on the surface of the conical cutter. The conical cutters cut through the earth when the weight of the drill pipe above the drill bit and the rotation of the drill pipe causes the conical cutters to independently rotate about their individual journals and cut through the earth.
Unlike oil field drilling where the drill bit will generally be cutting in the presence of liquid drilling mud, the blast hole drilling environment is dry and abrasive. In order to reduce interior wear of the oil field drill bit, fluid carrying conduits interior to the leg members and extending to the bearings inside the conical cutters supply lubrication to the bearings. In order to prevent loss of lubricant, typically each conical cutter of the oil field drill bits will have some sort of sealing means to retain the lubricant. The sealing means, which is located at the open end of the conical cutter recess, also prevents abrasive materials from entering through any space between the leg of the drill and the open end of the conical cutter mounted on the journal to the inside of the conical cutter to the bearings. However, since in blast hole drilling the environment is much more abrasive, the lubricating system and sealing means of an oil field drill bit are not used in many blast hole drill bits because the abrasive environment will quickly erode the seals, causing lubricant loss and drill bit failure.
Instead of supplying lubrication to the bearings to reduce wear, many drill bits circulate air through the bearings to cool the bearings and to wash away abrasive debris. For example, U.S. Pat. No. 3,921,735, by Dysart, discloses a passageway that extends through a bit body for conducting a gaseous drilling fluid to cool and clean the bearings. A cone mouth air screen is provided to screen out drilling debris. The screen material is selected so that its porous area is such that it will allow passage of all the air needed to cool and clean the bearings with a minimum of back pressure, but still fit satisfactorily into the minimal available space at the constricted cone mouth of a typical cone arm sub-assembly for a three cutter blast hole bit.
Sealing in lubricant is an effective way of lubricating the bearings and extending the life of the drill bit. There are attempts at providing sealed in lubricant for blast hole bits with means for protecting the seal from erosion from the abrasive blast hoe environment. U.S. Pat. No. 4,183,417, by Levefelt, discloses a rotary roller bit for drilling earth and rock formations which has a sealed lubrication system. The objective disclosed in that patent is to provide a barrier of air in the narrow space between the leg of the bit and the roller cutter to protect the seal from being damaged from debris. The periphery of each roller cutter is spaced from the adjacent portion of each leg so as to provide a jet slot for the discharge of air. Adjacent the slot and radially inwardly is an air chamber which is formed between an annular surface of the leg and the seal. Air is supplied through a passageway and is delivered to the air chamber and discharged from the chamber in a jet stream from the jet slot. The axial dimension of the air chamber is substantially greater than that of the slot so that the cross-sectional dimension of the path of air flow is substantially restricted as the air passes from the air chamber so as to produce a jet effect in the air flow. The air jet flowing from the jet slot is designed to prevent the entry of debris to the seal. However, one problem is that the jet slot has a dimension which is large enough for rock particles or other debris to enter. Furthermore, since the air from the air supply passageway passes into the air chamber and escapes from the jet slot wherever there is less resistance to air flow, this might create erratic air flow resulting in channeling and reverse flow conditions.
It is an object of this invention to provide a new and improved rotary blast hole bit which is sealed to retain lubricant for the bearings. Another object of the invention is to provide homogeneous air dissipation around the entire cone mouth to blow away borehole debris efficiently and uniformly, preventing air flow which results in channeling or reverse flow conditions, thereby protecting the sealing of the drill bit. It is a further object of this invention to use the homogeneous air dissipation around the cone mouth to protect an improved sealing arrangement.